Gas turbine engines are used for generating power in a variety of applications including land-based electrical power generating plants. Turbine engines produce power by extracting energy from a flow of hot gas produced by combustion of fuel and air in a combustion chamber (“combustor”) of the turbine. These hot gases are directed over rotatable blades to produce mechanical power before being released into the atmosphere. Turbine engines may be designed to combust a broad range of hydrocarbon fuels, such as natural gas, kerosene, diesel, etc in the combustor. Combustion of hydrocarbon fuel results in the production of combustion byproducts, some of which are considered regulated emissions. These regulated emissions include various forms of nitrogen oxides, collectively known as NOx. In an effort to reduce the emission of NOx to the atmosphere, government regulations limit the allowable emissions of NOx from turbines.
It is known that NOx emissions from turbine engines increase significantly as the combustion temperature rises. One method of limiting NOx in turbine exhaust is by using a lean mixture of fuel and air (low fuel-to-air ratio) in the combustor. A lean fuel-air mixture reduces the combustion temperature to a degree that reduces NOx production. While lean fuel-air mixture reduces NOx emissions, reducing fuel content in the mixture below a threshold value may cause the resulting flame in the combustor to be unstable. Instability of the combustion flame may result in the development of dynamic pressure waves in the combustor. These dynamic pressure waves may range in frequency from a few hertz to a few thousand hertz and occur as a result of the combustion process. These pressure pulses can result in mechanical damage to turbine components and smothering of the flame in the combustor (“lean blow-out”). Increasing the concentration of fuel in the mixture of fuel and air may stabilize the combustion process and reduce (or eliminate) harmful pressure pulses. The increased concentration of fuel may increase the temperature and heat release rate of the resulting flame leading to stabilization of the combustion process. This approach may, however, exacerbate the problem of controlling NOx production. Therefore, there must be a balance between the concerns of reduced emissions and stable combustion.
U.S. Pat. No. 6,877,307 issued to Ryan et al. ('307 patent) describes a method of controlling the combustion process of a turbine engine by increasing fuel to the combustor to achieve stable combustion. The method of the '307 patent uses a sensor to detect pressure pulses within a combustor. When the sensor detects pressure pulses above a threshold value, fuel flow to the combustor through the pilot is increased by a slight amount. Increasing fuel flow through the pilot increases NOx emissions. Combustor pressure monitoring is continued and the pilot fuel flow is gradually increased to a level at which the pressure pulses are below the threshold value. The method of the '307, thus, stabilizes the combustion process (by eliminating pressure pulses above a threshold value in combustor) by gradually increasing the pilot fuel to a value that is just enough to stabilize the combustion process. Although the combustion control system of the '307 patent may eventually stabilize the combustion process while increasing NOx emission to just the amount needed to achieve stable combustion, the system may have drawbacks. For instance, the gradual increasing of pilot fuel to achieve stable combustion, as disclosed in the '307 patent, may extend the amount of time the turbine engine operates in an unstable condition, and thus increase the potential for damage to the turbine.